Calcium is a key intracellular messenger, but excessive levels of Ca 2+ are toxic. This work addresses Ca 2+ signaling in glutamatergic axospinous synapses in the CA1 region of the rat hippocampus. Specifically: 1. To provide structural evidence on the extent of interactions among Ca 2+ sources in dendritic spines, the spatial organization of NMDA receptors and voltage-dependent calcium channels will be determined in individual dendritic spines. 2. Since the enzymatic activation of CaMKII is likely to be sensitive to its location in relationship to Ca 2+ microdomains, the organization of CaMKII within the spine will be studied. 3. Restoration of [Ca2+]i to resting levels is important not only to maintain the fidelity of Ca 2+ signals, but also to avoid the potential cytotoxicity associated with excessive calcium loads. Therefore, the organization of Ca 2+ pumps likely to be concentrated in the plasma membrane and organelles of the spine will be studied, as will calcium-binding proteins that may contribute to shaping Ca 2+ transients. Quantitative anatomical data generated by this work will be used to refine computer simulations running under the MCell program, allowing better insight into the functional significance of dendritic spines.